[TextAPI] update interface file for filtered iter

Summary:
This is a simple change that allows easy iterator semantics for symbols held in interface file.
Not being used, so harmless change right now, but will be once TBD-v4 is submitted.

Reviewers: ributzka, steven_wu

Reviewed By: ributzka

Subscribers: javed.absar, kristof.beyls, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67204

llvm-svn: 371097